The present invention relates to an improved automatic transmission and a method for adjusting shift timing in an automatic transmission. The invention is particularly advantageous for improving the shift timing during a shift from second to third gear.
The clutch of an automatic transmission in a 4, 6 or 8-cylinder vehicle may be damaged by improper timing during any shifts. Timing of such shifts is controlled by the flow of transmission oil between the servo, shift valves and clutch. During a shift from second to third gear, for example, the 3-4 clutch must apply as the 2-4 band releases. If transmission oil does not flow to the clutch and the release side of the 2-4 servo at properly controlled ratios, damage will occur.
A schematic diagram of prior art THM 700-R4 or 4L60-E transmissions 70 is provided in FIG. 3. In these prior art transmissions, as the transmission shifts from second to third gear, transmission oil flows through an orifice 28 plus unseated check ball 30, between the 3-2 control valve and the 2-4 servo toward the release side of the 2-4 servo. Both the 3-4 apply oil and 2-4 band release oil must pass through a single orifice 14 from the manual valve 12. Therefore, the apply of the 3-4 clutch and release of the 2-4 band do not occur smoothly and in proper timing at varying vehicle speeds. The failure rate for 3-4 clutches in these THM transmissions is known to be excessive and has been estimated to be as high as 35%.